1. Field of the Invention
This invention relates generally to horizontal deflection circuits and, more particularly, to a horizontal deflection circuit for use with a multiple-scanning type television receiver that can receive video signals having different horizontal frequencies.
2. Description of the Background
Recently, a so-called multiple-scanning type television receiver has been developed. This multiple-scanning type television receiver required a horizontal deflection circuit that can provide a constant horizontal deflection regardless of a change in the horizontal or line frequency of the video signal input for display. An example of a proposed horizontal deflection circuit for use in a multiple-scanning type television receiver is shown in FIG. 1. In that example, a horizontal synchronizing signal is applied to an input terminal 1 and then fed to a frequency-to-voltage converter 2, where it is converted to a voltage signal whose level is changed in proportion to the frequency of the horizontal synchronizing signal. The voltage signal from frequency-to-voltage converter 2 is supplied to one input of a multiplier 3.
A second input terminal 4 has supplied thereto a deflection control signal that is used to vary the width of deflection of the beam in the horizontal direction. The deflection control signal applied to input terminal 4 is supplied to the other input terminal of multiplier 3, so that the voltage signal corresponding to the frequency of the horizontal synchronizing signal is changed in response to the deflection width indicated by the deflection control signal. The product signal from multiplier 3 is supplied to a switching regulator 5 as a control signal, and switching regulator 5 produces a DC output signal in response to the multiplied signal. This DC output signal is supplied to a horizontal output circuit 6 as a power supply source signal, and the horizontal synchronizing signal at input terminal 1 is also supplied to horizontal output circuit 6, whereby a horizontal deflection output signal having a cycle based on the horizontal synchronizing signal is developed at an output terminal 6a, which is connected to the deflection coil (not shown) of the television receiver.
According to the above-described horizontal deflection circuit, in response to a change in the horizontal frequency of the input video signal the frequency of the horizontal synchronizing signal at input terminal 1 will change accordingly, whereby the power supply source voltage fed to horizontal output circuit 6 from switching regulator 5 will also change. Therefore, the horizontal deflection width can be controlled substantially to fall in a predetermined range, regardless of changes in the line frequency of the video input signal.
Alternatively, the horizontal deflection circuit may be constructed as shown in FIG. 2, in which like parts corresponding to those of FIG. 1 have the same reference numerals and need not be described in detail.
In FIG. 2, the horizontal synchronizing signal applied to horizontal synchronizing signal input terminal 1 is again supplied directly to horizontal output circuit 6, and the output signal from horizontal output circuit 6 is supplied to a deflection width detecting circuit 7. Deflection width detecting circuit 7 detects a horizontal deflection width from the output signal of horizontal output circuit 6 and produces a output signal that is integrated by an integrating circuit 8 and is then fed to one input of a comparator 9. The electronic control signal that is proportional to the deflection width is again applied to deflection control signal input terminal connected to the other input of comparator 9. Comparator 9 compares the deflection width indicated by the control signal with a real deflection width indicated by the detected signal to generate a compared result, that is, a difference signal. This difference signal from comparator 9 is supplied to switching regulator 5 as a control signal, and a DC signal from switching regulator 5 is again supplied to horizontal output circuit 6 as a power source voltage. Deflection width detecting circuit 7 operates so that a horizontal deflection pulse signal derived from horizontal output circuit 6 is peak-rectified, thereby detecting the deflection width from a rectified value.
The method in which the horizontal deflection pulse is peak-rectified to detect the deflection width, however, cannot avoid the disadvantage that when the line frequency fluctuates the value to be detected also fluctuates, thereby causing an error in the detected value. This error condition will be described more fully with reference to the waveform diagram of FIG. 3.
If it is assumed that a horizontal deflection pulse signal generated from the horizontal output circuit is shown by a dashed line in FIG. 3, then a peak-rectified value P.sub.1 of the horizontal deflection pulse signal will change in a predetermined range near the peak value of the pulse signal, as shown by a solid line in FIG. 3. In that case, the mean value level of the peak-rectified value P.sub.1 is represented by level a.sub.1 in FIG. 3. Assuming that the horizontal frequency of the incoming video signal is reduced by one half, then the interval between the horizontal deflection pulse signals is widened by a factor of two, whereby the width in which a peak-rectified value P.sub.2 is changed is widened to twice that of the peak-rectified value P.sub.1. As a result, the mean value level a.sub.2 will be lower than the mean value level a.sub.1. When the mean value is lowered as described above, even though the real deflection width is not changed, it is detected as if the deflection width had changed, thereby producing an error in the detected value.
With recent developments in broadcasting techniques, the horizontal frequency of video signals has become higher and higher. In fact, such horizontal frequency is as high as several times the horizontal frequency according to the ordinary broadcasting standard (15.75 kHz). If a video signal with such a high horizontal frequency is input to the detecting circuit, there is then the substantial possibility that an error will occur in detecting the deflection width.
In accordance with the horizontal deflection circuit of FIG. 2, as in the example of the horizontal deflection circuit of FIG. 1, regardless of a change in horizontal frequency of the video signal, the horizontal deflection width can be substantially controlled to fall in a predetermined range. More particularly, the deflection width to be controlled and the real deflection width are compared by comparator 9, and switching regulator 5 is controlled so as to reduce the resultant difference, whereby the horizontal deflection width can be prevented from fluctuating with changes in horizontal frequency.
Nevertheless, the horizontal deflection circuit shown in FIG. 1 has the unavoidable disadvantage that the horizontal deflection width fluctuates very slightly in accordance with an increase in horizontal frequency f.sub.H of the incoming video signal.
More specifically, although the horizontal frequency f.sub.H and the output voltage V of the switching regulator 5 are directly proportional to each other, as shown by a dashed line in FIG. 4, in practice the output voltage V increases at a higher rate in accordance with increases of the horizontal frequency f.sub.H, as shown by a solid line in FIG. 4, resulting in the horizontal deflection width fluctuating by about 10 to 20%. The reason that the horizontal deflection width fluctuates is that even though the horizontal frequency changes the beam blanking period, or retrace period, does not change. This will be described in more detail with reference to FIGS. 5A and 5B.
In a signal supplied to a horizontal deflection coil during one horizontal scanning period H at a certain horizontal frequency, as shown in FIG. 5A, the voltage value is linearly increased during a video scanning period T.sub.1, whereas the voltage value is decreased during the retrace blanking period T.sub.R. Let it be assumed that, as shown in FIG. 5B, the horizontal frequency is doubled thereby reducing one horizontal scanning period H.sub.2 to half of the above-described one horizontal scanning period H.sub.1. Nevertheless, blanking period T.sub.R is necessarily constant regardless of the horizontal frequency, so that the video scanning period T.sub.2 of the horizontal scanning period H.sub.2 must become shorter than half of the video scanning period T.sub.1 of the horizontal scanning period H.sub.1. Therefore, the change in the horizontal frequency and the change of the video scanning period are not directly proportional to each other, so that in the horizontal deflection circuit of FIG. 1 an error occurs in the deflection width in accordance with the increase of the horizontal frequency, thereby a fluctuation in the deflection width occurs.
Further in the horizontal deflection circuit of FIG. 2 wherein the deflection width is detected and a detected difference is fed back to switching regulator 5 in a feedback loop, although an error can be prevented from being produced in the deflection width due to the horizontal frequency, an integrating circuit is needed to reduce the cut-off frequency in order to prevent the circuits forming the loop section from oscillating. Nevertheless, if the cut-off frequency is reduced as described above, then the response characteristic of switching regulator 5 deteriorates and switching regulator 5 cannot follow rapid changes in the horizontal frequency f.sub.H. There is then the substantial risk that horizontal output circuit 6 will be damaged.